Psoriasis is a common chronic disease affecting around 2% of the general population and any person from infancy to old age. Its etiology and pathogenesis are unclear. Family and twin studies have suggested a polygenic influence but the nature and the mechanisms of action of the involved genes are unknown (Elder J T at al. (2001) Arch. Dermatol. 137:1447-1454). Environmental factors such as streptococcal infections and trauma to the skin are also associated with the formation of psoriatic lesions. How do these environmental factors contribute to psoriasis are again unclear. However the association with streptococcal infections, immunosuppressive actions of most of the current anti-psoriatic treatments and other findings are used widely to argue for an autoimmune nature of the disease and triggering by T Lymphocytes (Gottlieb S. L. et al (1995) Nat. Med. 1:442-447; Nickoloff B. J. (1999) Arch. Dermatol. 135:1104-1110; Krueger J. G. (2002) J. Am. Acad. Dermatol. 46:1-23).
Psoriasis vulgaris, characterized by well-demarcated scaly erythematous plaques of varying sizes anywhere on the skin, is the most common form of psoriasis. Histopathological examinations of the psoriatic skin lesions reveal typical epidermal and dermal changes that include the following.
Epidermal hyperplasia with elongation and thickening of rete ridges.
Thinning of the suprapapillary epidermis.
Focal losses or decrease of the thickness of the granular layer of epidermis.
Infiltration of the subepidermal region of dermis with neutrophils and mononuclear inflammatory cells.
Dilatation and tortuosity of the capillaries in the papillary dermis, accompanied often by papillary edema.
“Munro microabcesses”, defined as focal intracorneal collections of neutrophils.
Psoriatic skin lesions generally contain majority of the above listed histopathological changes and all changes can be found in a well-developed psoriatic lesion. In addition immunohistochemical and other indicators of the proliferating cells reveal presence of proliferating keratinocytes in the suprabasal layers of psoriatic lesional skin (proliferating cells are normally restricted to the basal layer of epidermis in healthy skin).
Obscurity of the etiology and pathogenesis of psoriasis has been reflected by the varied treatment strategies used for this disease (Spuls P. I. et al. (1997) Br. J. Dermatol. 137:943-949; Ashcroft D. M. et al. (2000) J. Clin. Pharm. Ther. 25:1-10; AI-Suwaidan S. N. et al. (2000) J. Am. Acad. Dermatol. 42:796-802; Lebwohl M. et al. (2001) J. Am. Acad. Dermatol. 45:487-498; Lebwohl M. et al. (2001) J. Am. Acad. Dermatol. 45:649-661). Currently common treatments include the topical corticosteroids, systemic administration of immunosuppressants (usually cyclosporine), ultraviolet irradiation of the affected skin with or without psoralen, systemic retinoids and systemic methothrexate (Spuls P. I. et al. (1997) Br. J. Dermatol. 137:943-949; Ashcroft D. M. et al. (2000) J. Clin. Pharm. Ther. 25:1-10; Lebwohl M. et al. (2001) J. Am. Acad. Dermatol. 45:487-498; Lebwohl M. et al. (2001) J. Am. Acad. Dermatol. 45:649-661). At present there is no cure for psoriasis and patients face a need for life-long treatment. Therefore relatively simpler treatments (usually topical keratolitics and corticosteroids) are considered first and when these fail, the more effective systemic treatments with more serious side effects are attempted. When the therapeutic aim is defined as the clearance of lesions, even the most effective systemic treatments are reported to fail in as many as one fourth of patients in large series (Spuls P. I. et al. (1997) Br. J. Dermatol. 137:943-949) and, because of the serious side effects, patients and physicians are advised that at present “complete clearance is not a realistic expectation” (AI-Suwaidan S. N. et al. (2000) J. Am. Acad. Dermatol. 42:796-802). In practice side effects usually limit the more potent treatments to shorter-term management [cyclosporine is nephrotoxic and strongly immunosuppressive, methotrexate is hepatotoxic, ultraviolet irradiation-psoralen is mutagenic/carcinogenic (Lebwohl M. et al. (2001) J. Am. Acad. Dermatol. 45:649-661)]. However, in the long term, topical carticosteroids are also not devoid of side effects (Lebwohl M. et al (2001) J. Am. Acad. Dermatol. 45:487-498). Currently available treatments require in general several weeks (typically 6-8 weeks) from the initiation of treatment to the appearance of objective clinical regression (AI-Suwaidan S. N. et al. (2000) J. Am. Acad. Dermatol. 42:796-802; Lebwohl M. et al. (2001) J. Am. Acad. Dermatol. 45:649-661).
Cyclopamine is a steroidal alkaloid that occurs naturally in the Veratrum plants. Teratogenicity of these plants on grazing pregnant animals led to the identification of cyclopamine as an active compound (Keeler R. F. (1969) Phytochemistry 8:223-225). How might have cyclopamine displayed teratogenicity was revealed by the finding that it is an inhibitor of the hedgehog/smoothened signal transduction pathway (Incardona J. P. et al. (1998) Development 125:3553-3562; Cooper M. K. et al. (1998) Science 280:1603-1607). The sonic hedgehog protein, a member of the hedgehog family of proteins, has been found to induce differentiation of its target cells, including the precursors of ventral cells in the developing central nervous system (Goodrich L. V. et al. (1998) Neuron 21:1243-1257). Inhibition of the hedgehog/smoothened pathway by cyclopamine in the developing chicken brain prevented formation of the ventral cells and caused holoprosencephaly (Incardona J. P. et al. (1998) Development 125:3553-3562; Cooper M. K. et al. (1998) Science 280:1603-1607), the common malformation observed in the lambs of the sheep grazing Veratrum (Binns W. et al. (1963) Am. J. Vet. Res. 24:11641175). Cyclopamine has been reported to inhibit cellular differentiation in other systems as well, including the differentiation of bone marrow cells to erythroid cells (Detmer K. et al. (2000) Dev. Biol. 222:242) and the differentiation of the urogenital sinus to prostate (Berman D. M. et al. (2000) J. Urol. 163:204).